i used to have abit IC7-G and i had the same problem as you do.
i was using it with Kingston KHX4300. and for some reason, i can only do FSB250. and i am not even overclock the memory (run 5:4) anything beyound 250 will crash. even 251.

then i bought KHX4000. and surprisingly, it seems ok now. i can go up to around 280( and i guess my cpu is limit factor here)

ps. KHX use winbond CH5 chip. so i guess IC7 doesn't like ch5
KHX4000 use hynix chip.